Flexible unbonded pipes of the present type are for example described in the standard “Recommended Practice for Flexible Pipe”, ANSI/API 17 B, fourth Edition, July 2008, and the standard “Specification for Ubonded Flexible Pipe”, ANSI/API 17J, Third edition, July 2008. Such pipes usually comprise an inner liner also often called an inner sealing sheath or an inner sheath, which forms a barrier against the outflow of the fluid which is conveyed in the bore of the pipe, and one or more armoring layers. In general flexible pipes are expected to have a lifetime of 20 years in operation.
Examples of unbonded flexible pipes are e.g. disclosed in WO0161232A1, U.S. Pat. No. 6,123,114 and U.S. Pat. No. 6,085,799.
The term “unbonded” means in this text that at least two of the layers including the armoring layers and polymer layers are not bonded to each other. In practice the known pipe normally comprises at least two armoring layers located outside the inner sealing sheath and optionally an armor structure located inside the inner sealing sheath normally referred to as a carcass. These armoring layers comprise or consist of multiple elongated armoring elements that are not bonded to each other directly or indirectly via other layers along the pipe. Thereby the pipe becomes bendable and sufficiently flexible to roll up for transportation.
In prior art pipes, these armoring layers are largely torsionally balanced. To ensure sufficient torsional balance, at least two of the armoring layers located outside the inner sealing sheath are counter wounded at approximately the same angle relative to the centre axis of the pipe.
A pipe of the above type will for many applications need to fulfill a number of requirements. First of all the pipe should have a high mechanical strength to withstand the enormous forces it will be subjected to during transportation, deployment and operation. The internal pressure (from inside of the pipe and outwards) and the external pressure (from outside of the pipe) are usually very high and may vary considerably along the length of the pipeline particular when applied at varying water depths. If the internal pressure of the pipe exceeds the load bearing capacity of the armoring elements of the armoring layers located outside the inner sealing sheath the internal pressure will ultimately result in damage of the pipe e.g. by upheaval buckling and/or burst of the flexible pipe. If the pipe resistance against the external pressure is too low, the external pressure may ultimately result in catastrophic failure of one or more of the armoring layers resulting in a subsequent failure of the inner sealing sheath and ultimate failure of the flexible pipe e.g. due to collapse of the inner sealing sheath which acts as the primary barrier against outflow of a fluid transported in the flexible pipe. Simultaneously with such severe mechanical loadings the flexible pipe may be subjected to highly corrosive fluids and chemical resistance may be needed. Furthermore, it is often desired to keep the weight of the pipe relatively low, both in order to reduce transportation and deployment cost but also in order to reduce the risk of damaging the pipe during deployment.
In traditional flexible pipes, the armoring layers often comprise metallic armoring layers including a metal carcass typically wound from preformed or folded stainless steel strips and a number of armoring layers in the form of helically wound profiles or wires, where the individual layers may be wound with different winding angles relative to the pipe axis in order to take up the forces caused by internal and external pressure as well as forces acting at the ends of the pipe and shear forces from the surrounding water.
When subjected to hydrostatic pressure in the sea the carcass of the prior art pipe will usually be designed to be sufficiently strong to withstand the hydrostatic pressure, and the armoring layers in the form of helically wound profiles or wires should be designed to be sufficiently strong to withstand internal pressure and tearing in the length direction of the pipe.
In the prior art it has been suggested to replace one or more of the metallic armoring layers with armoring layers of fibers or fiber reinforced polymer of different structures. U.S. Pat. No. 6,165,586 for example discloses a strip of filamentary rovings sampled with bonding material and retaining means. It is suggested to use such strips to replace one or more metallic armoring layers of an unbonded flexible pipe.
WO 01/51839 discloses a flexible unbonded pipe comprising a tensile reinforcement layer of fiber embedded in a polymeric material.
Several older publications disclose fiber reinforced pipes where all the armoring layers are provided by fibers wound in various winding angles. Examples of such pipes are described in U.S. Pat. No. 5,110,644 and U.S. Pat. No. 4,431,034.
However, these prior art pipes are either very heavy due to metal armoring layers or they are very complicated to produce and/or have a very low burst resistance.